Surface: The surface of these planets was abundant in water. Under the surface, there were additional sources of water. These worlds were geologically active. (VOY: "Caretaker", "Once Upon a Time", "Dragon's Teeth")

During the mid-22nd century, Vulcan science used the term Minshara class for such planets. Vulcans were not able to determine if a planet was Minshara-class through orbital scans, instead sending down probes to collect the necessary data. (ENT: "Strange New World") In 2151, Starfleet officers were not familiar with the term and Enterprise chose to adopt it. However, by 2154, Starfleet had adopted the term class M. A century later, that term was in general use in Starfleet, even though the term "Earth-type" was also occasionally used.

The term "class M" was first used in "The Cage" to describe the planet Talos IV. The variant "M class" was also often used. The term has become Star Trek shorthand for Earth-like, and due to obvious production practicalities is the "default" for planetary surface scenes.

The Vulcan term "Minshara-class" (first used in ENT: "Strange New World") was used in Enterprise to denote planets that in other series would have been called class M by the writers, the implied consequence being that the two terms meant the same, and possibly even that M stood for Minshara. This assumption has been contested by some fans, but is at least supported by the reference bookStar Trek: Star Charts. From an in-universe standpoint, the term M-class was first seen chronologically in a text within the Handbook of Exobiology in "Strange New World", the same episode that introduced Minshara-class. However, this mention was barely legible on-screen and may have been included by an art department not yet aware of the intention to use "Minshara class". The first spoken use of the word was in ENT: "Home", in which Archer used it to describe Archer IV, a planet implied (but not confirmed) to be Minshara class in ENT: "Strange New World". The term also appeared on Enterprise's computer displays, indicating that Starfleet adapted it eventually.

In TOS: "Return to Tomorrow", Spock called planet Arret class M despite the atmosphere having been ripped away, suggesting that the class M designation goes beyond being able to support life. Alternately, he could have been saying that the planet, prior to losing its atmosphere, would have been categorized as class M.

Ceti Alpha V was described as "barely Minshara class" in "Twilight", but also as no more inhospitable and savage then the site of Australia's Botany Bay colony before colonization in "Space Seed", suggesting that even on a class M planet like Earth, there might be locations that stretch the definition.

According to Star Trek: Star Charts, class M planets have ages that range from 3 billion to 10 billion years and a diameter between 10,000 and 15,000 kilometers. They are located within the ecosphere of a star system.